The present invention relates to methods an apparatus for determining the concentration of tissue pigments, such as hemoglobin, using time-resolved pulses of light. The present invention relies upon the decay characteristics associated with absorptive pigments to derive their concentrations.
The utility of optical methods for studying metabolism and oxidative processes in cells and tissues was significantly enhanced in the early 1950""s when time-sharing dual-wavelength systems were developed for the quantitation of small changes in the absorption of light in the visible and near-infrared (NIR) regions in highly scattering media, such as cell suspensions or muscle tissues. See Chance, B. Rev. Sci. Instrum. 22,619-627 (1951); Chance B. Biochemistry of Copper, ed. Peisach, J. (Academic, New York), pp. 293-303 (1966), which are incorporated by reference as if fully reproduced herein. Fluorescence signals from mitochondrial NADH complemented the absorption method for studies of the surface of heart, brain, and skeletal tissue. See Chance, B., et al., Nature (London) 195, 1073-1075 (1962) and Chance B. Science 120, 767-775 (1954). NIR spectroscopy has been used to detect the redox state of the copper component of cytochrome oxidase in mitochondria and yeast cells. See Chance, B. Biochemistry of Cooper, ed. Peisach, J. (Academic, New York), pp. 293-303 (1966); Chance, B. J. Biol. Chem 234, 3036-3040 (1959). Jobsis-Vander Vliet and coworkers pioneered the study of NIR absorption in tissue by transillumination. See Jobsis-Vander Vliet, F. F. Adv. Exp. Med. Biol. 191, 833-842 (1985); Jobsis-Vander Vliet, F. F., et al., J. Appl. Physiol. 113, 858,872 (1977); see also Rosenthal, M., et al., Brain Res. 108, 143-154 (1976). Algorithms have been developed to compensate for the interference from hemoglobin and myoglobin with cytochrome copper, as the latter may constitute as little as 5% of the total signal at 830 nm. See vanderZee. P. and Delpy, D. T. (1988) Oxygen Transport to Tissue X, eds. Mochizuki, M., et al., (Plenum, New York), pp. 191-197); see also Tamura, M. H., et al., Chemoreceptors and Reflexes in Breathing, ed. Lahiri, S. (Oxford, New York), in press.
An application of the principles of spectrophotometry is the use of continuous light (CW) illumination to determine the attenuation characteristic of light in systems containing localized deoxyhemoglobin (Hb) and to observe hypoxia in the brain. However, although CW systems can be used to quantify changes in optical density, they cannot quantify the concentration of an absorptive pigment.
The change in intensity of light, or the optical density (log Io/I), in an absorbing medium generally follows the Beer-Lambert Law:       log    ⁢          xe2x80x83        ⁢                  I        o            I        =  ECL
where I is the intensity of light, E is the extinction coefficient, C is the concentration of an absorptive pigment in the medium, L is the optical path length, and the absorption per unit length is defined as u=EC=1/L (log Io/I). This law is the basis for modern spectrophotometry and, as well known to those of ordinary skill, has been repeatedly verified by both research and in industrial applications. The Beer-Lambert Law requires the determination of at least one of two quantitiesxe2x80x94specific absorption or path lengthxe2x80x94in addition to intensity. Since CW systems can only quantify changes in intensity, only the product uL can be determined.
When light is directed into an absorptive medium such as a tissue region, it is scattered, reflected, or it migrates from one point to another by a random walk or other diffusion process. The actual length traversed may be the distance between the light input and output in a rectilinear direction, since little or no light is believed to move in an orthagonal direction, except where florescence or other processes may be involved. Under these conditions, CW systems thus cannot independently quantify the specific absorption for a given medium. Therefore, the determination of the concentration of an absorptive pigment must necessarily be carried out by arbitrary calibrations using media of known values of CE in order to determine the relationship between optical density (OD) and path length (L). After L has been determined, the system can then be used to determine subsequent variations in the concentration of the absorptive pigment within a tissue region. This is the general method by which CW absorption spectrometry is carried out with scattering materials. Often, information concerning L is obtained at an adjacent wavelength, such as an isosbestic point. This method is particularly effective when two values of L are similar leading to what is termed precise dual wavelength spectrophotometry of biological tissue. See, for example, the system disclosed in U.S. patent application Ser. No. 266,166, filed Nov. 2, 1988, entitled, xe2x80x9cOptical Coupling System For Use In Monitoring Oxygenation State Within Living Tissuexe2x80x9d, which is incorporated herein by reference as if fully reproduced herein.
Difficulties arise, however, when C for an absorptive material cannot be varied in a rational or known manner. For example, in living tissues, a known change in concentration, C, of an absorptive pigment such as hemoglobin may be difficult to impose, and if imposed, may result in traumatic consequences to the subject. In the case of limbs, tourniquet ischemia can be used to cause a known change in the concentration of hemoglobin, but individual variations and physiological adaptations may falsify any calibrations made. In the brain and heart, where oxygenation data are most needed, it is not possible to calibrate by large perturbations of the system, since damage or death might result. Under these conditions, it is difficult to calibrate the meaning of the trends in observations that may occur in some diseases such as neonatal hypoxia or adult stroke. Thus, the calibration method described loses its force and reliability. Similarly, although animal models have been resorted to for purposes of calibration, since the boundary conditions for photon migrationxe2x80x94which are crucialxe2x80x94are likely to differ between the animal model and a human subject, these methods have also generally proven unsatisfactory.
Accordingly, it has now been found that the concentration of an absorptive pigment in a scattering medium can be quantitatively determined, thereby allowing the path length within the medium to be calibrated. The present invention discloses methods of determining the concentration of a tissue pigment with a known absorption spectrum by illuminating a portion of a tissue region at a first location with a pulse of light having a known duration. The migration of the pulse of light through the tissue region to a second location remote from the first location is then detected, and occurs as a rise to a maximum value and thereafter a decay in the intensity of light at the second location. The rate of decay of the light at the second location is determined; and is expressed as:xe2x88x921/(L (log I)). This quantity is proportional to the concentration of a tissue pigment with a known absorption spectrum. Finally, the rate of decay, u, is divided by the extinction coefficient (E) to determine the concentration (C) of a particular tissue pigment with a known absorption spectrum.
The present invention is preferably used to determine the concentration of tissue pigments with known extinction coefficients such as hemoglobin, oxyhemoglobin, deoxyhemoglobin, or myoglobin.
In a preferred embodiment, the duration of the input pulse of light is less than about one nanosecond, and most preferably is about 6 picoseconds.
The methods of the present invention are useful for determining the concentration of tissue pigments in living tissue, and further, allow the concentration to be determined substantially instantaneously.
In use, the methods of the present invention preferably place the input pulse and output detector in proximity with the outer surface of the tissue region being studied. Most preferably, the location of the output detector is displaced from the location of the input pulse by a distance of at least several centimeters. Thus, the input pulse enters the tissue region and migrates to the adjacent output detector.
The methods of the present invention preferably include displaying the concentration of a tissue pigment with a known absorption spectrum as an absolute value from which the oxygen content of said region of tissue is determined.
The present invention also discloses apparatus comprised of a laser system capable of generating short pulses of light, transmitting them to the subject, detecting the photons after they have migrated through the subject, determining the rate of decay of the photons and processing the data collected into a final data set representing the concentration of an absorptive pigment having known absorptive characteristics.